Make blocks callable

[ixjlyons]

In some cases, it would be better to simply treat blocks as if they were functions. Many blocks really should just be functions anyway, since they don't need to carry state. This would also make CallablePipelineBlock totally unnecessary.

A simple solution would be to just add this to PipelineBlock:

def __call__(self, *args, **kwargs):
    return self.process(*args, **kwargs)

A better solution would probably be to just get rid of the process method altogether.

[ixjlyons]

Addressed for now with bd67c67. Not closing for now -- I'd like to think a bit more about this plus it should be documented somewhere.